Aluminum has an excellent conductive property and is used as an electrode material of a battery such as a lithium-ion battery. For example, the positive electrode of a lithium-ion battery is constituted by an aluminum foil to the surfaces of which an active material such as lithium cobalt oxide is applied.
To increase the capacity of such a positive electrode, a porous aluminum body can be used so that the surface area of the positive electrode is increased and the aluminum body is filled with an active material; in this case, the active material is available even when the thickness of the electrode is large, and hence the availability ratio of the active material per unit area increases.
Such a porous aluminum includes an aluminum nonwoven fabric formed by entanglement of fibrous aluminum and an aluminum foam formed by foaming of aluminum. Patent Literature 1 discloses a method for producing a metal foam containing a large number of independent bubbles by adding a foaming agent and a thickening agent to a molten metal and stirring the resultant mixture.
As a porous metal, there is a porous nickel body that is commercially available under the trade name Celmet (registered trademark). Celmet (registered trademark) is a porous metal body that has continuous pores and has a high porosity (90% or more). This is obtained by forming a nickel layer on the surface of the skeleton of a porous resin body containing continuous pores such as a urethane foam, subsequently decomposing the porous resin body by a heat treatment, and subjecting the nickel to a reduction treatment. The nickel layer is formed in the following manner: the porous resin body is subjected to a conductive treatment by the application of carbon powder or the like to the surface of the skeleton of the porous resin body, and nickel is subsequently deposited on the porous resin body by electroplating.
Patent Literature 2 discloses a method for producing a porous metal body in which the production method of Celmet is applied to aluminum. Specifically, a film of a metal (copper or the like) that forms a eutectic alloy with aluminum at a temperature equal to or less than the melting point of aluminum is formed on the skeleton of a porous resin body having a three-dimensional network structure; the porous resin body is subsequently coated with aluminum paste; the resultant body is subjected to a heat treatment at a temperature of 550° C. or more and 750° C. or less in a non-oxidizing atmosphere to evaporate the organic constituent (porous resin body) and to sinter the aluminum powder to thereby provide the porous metal body. Patent Literature 2 states that, although aluminum forms a strong oxide film and hence has sintering resistance, aluminum powder applied on a film of a metal that forms a eutectic alloy with aluminum causes a eutectic reaction at the surface boundary between the aluminum powder and the metal film as a base in the process of a heat treatment to produce liquidus surfaces at a temperature equal to or less than the melting point of aluminum; and the partially produced liquidus surfaces breach the oxide film of aluminum so that sintering of the aluminum powder proceeds while the three-dimensional network skeleton structure is maintained.